1. Field of the Invention
The invention provides new methods for synthesis of isotopically enriched borohydride compounds and methods of preparing isotopically enriched neutral or anionic boranes using isotopically enriched borohydride prepared by the methods of the invention as an intermediate. More particularly, the present invention provides improved methods of synthesis of isotopically enriched metal borohydrides from isotopically enriched boric acid and methods of synthesis of isotopically enriched neutral or anionic boranes having between 5 and 96 boron atoms, which methods comprise preparing an isotopically enriched metal borohydride as an intermediate in the preparation of the neutral or anionic borane.
2. Background.
Large boron hydride compounds have become important feed stocks for boron doped P-type impurity regions in semiconductor manufacture. More particularly, high molecular weight boron hydride compounds, e.g., boron hydride compounds comprising at least a five (5) boron atom cluster, are preferred boron atom feed stocks for boron atom implantation.
An important aspect of modern semiconductor technology is the continuous development of smaller and faster devices. This process is called scaling. Scaling is driven by continuous advances in lithographic process methods, allowing the definition of smaller and smaller features in the semiconductor substrate which contains the integrated circuits. A generally accepted scaling theory has been developed to guide chip manufacturers in the appropriate resize of all aspects of the semiconductor device design at the same time, i.e., at each technology or scaling noted. The greatest impact of scaling on ion implantation process is the scaling of junction depths, which requires increasingly shallow junctions as the device dimensions are decreased. This requirement for increasingly shallow junctions as integrated circuit technology scales translates into the following requirement: ion implantation energies must be reduced with each scaling step. The extremely shallow junctions called for by modern, sub-0.13 micron devices are termed “Ultra-Shallow Junctions” or USJs.
Methods of manufacturing boron doped P-type junctions have been hampered by difficulty in the ion-implantation process using boron. The boron atom, being light (MW=10.8), can penetrate more deeply into a silicon substrate and diffuse throughout the substrate lattice rapidly during annealing or other elevated temperature processes.
Boron clusters or cages, e.g., boranes have been investigated as a feed stock for delivering boron to a semiconductor substrate with reduced penetration. For example, boron ions may be implanted into a substrate by ionizing boron hydride molecules of the formula BnHm (where 100>n>5 and m≦n+8) and an ion source for use in said implantation methods. Certain preferred compounds for use in the boron ion implantation methods included decaborane (B10H14) and octadecaborane (B18H22).
Large boron hydride compounds, that is boron compounds having between 5 and about 100 (more typically between 10 and about 100 or between 5 and about 25 boron atoms) are preferred for use in molecular ion implantation methods for delivering boron atoms to a semiconductor substrate. Typically two or more structural isomers exist of large boron hydride compounds, e.g., two or more compounds having the same chemical formula but different structural arrangement of boron atoms in the cage structure. In addition, two or more structurally related boron hydride compounds having the same number of boron atoms but different numbers of hydrogen atoms have been isolated for various sized boron clusters. Such compounds are frequently referred to as closo (BnHn), nido (BnHn+2), arachno (BnHn+4), hypho (BnHn+6), conjuncto (BnHn+8), and the like. Thus, a plurality of different boron hydride species, including structural isomers and compounds containing various amounts of hydrogen are frequently known for boron hydrides having n boron atoms. See, for example, Jemmis, et al., J. Am. Chem. Soc., v. 123, 4313-4323 (2001), which provides a review of various macropolyhedral boranes and known compounds having n boron atoms and various amounts of hydrogen.
International patent application WO 03/044837, (Applied Materials, Inc, Santa Clara Calif.) recites methods of ion implantation in which an isotopically enriched boron compounds including 11B enriched compounds are ionized and then implanted into a substrate. The '837 publication recites the preparation of the isotopically enriched boranes by the method recited in U.S. Pat. No. 6,086,837 (Cowan, et al.), which methods are reported to be the current industrial process for the preparation of boranes isotopically enriched in 10B or 11B.
Cowan (U.S. Pat. No. 6,086,837) recites a method of preparing B-10 enriched decaborane starting with B-10 enriched boric acid. The Cowan preparation of either B-10 or B-11 enriched boron hydrides begins with boric acid and involves a multitude of synthetic and purification steps. More particularly, the Cowan process for conversion of boric acid into an alkali metal borohydride involves numerous time consuming steps and results in a relatively low yield of valuable B-10 enriched borohydride which must then be subjected to further reactions to obtain final product.
Thus, the Cowan method starts with the preparation of B-10 methylborate from boric acid and methanol using an azeotropic distillation method. The methylborate is separated from remaining methanol by freeze recrystallization by means of three one step procedures to produce an 80% yield of trimethylborate. The trimethylborate is then added to a suspension of sodium hydride in mineral oil at 220° C.-250° C. and heated for 12 hrs. For safety, a metal reflux condenser is required. Isolation of the formed borohydride requires special attention. First, the excess sodium hydride is destroyed by pouring the mineral oil mixture into a mixture of ice and water, a rather exothermic process evolving gaseous hydrogen. Then the aqueous borohydride is separated from the mineral oil by decantation or use of separatory funnel. The aqueous borohydride must be purged of methanol by either heating to 60 C and purged with a nitrogen stream or by removal under reduced pressure. The resulting aqueous solution is comprised of sodium hydroxide and the B-10 enriched borohydride. Carbon dioxide gas is bubbled through the solution converting the sodium hydroxide to sodium carbonate. The resulting slurry is then extracted with n-propylamine and the n-propylamine evaporated to yield final product. The solubility of sodium borohydride in n-propylamine is limited and appreciable volumes of the volatile solvent are needed. Typical yields of 45-65% are obtained. A total of ten time consuming steps are required to prepare isotopically enriched sodium borohydride by the procedure recited in Cowan.
U.S. Pat. No. 2,642,453, issued to Lippincott, relates to methods of preparing borates of tertiary alcohols such as methods of preparing tri(tert-butyl)borate and the like by condensation of boric acid and a tertiary alcohol and water removal by fractional distillation of an azeotrope. The preparation and rate of hydrolysis of a variety of boric acid esters was recited by H. Steinberg and D. L. Hunter in Industry and Engineering Chemistry, v. 49, No. 2, (1957) p. 174-181.
U.S. Pat. No. 3,063,791, issued to Kollonitsch, et al., relates to method of preparing natural abundance alkali and alkaline earth metal borohydrides from boric acid by contacting an intermediate trialkylborate with alkali metal aluminum hydride. Kollonitsch does not provide methods of synthesis of isotopically enriched borohydrides.
Dunks and coworkers recite methods of preparing MB11H14 salts and decaborane (B10H14) from metal borohydride or MB3H8 starting materials. U.S. Pat. No. 4,115,520, U.S. Pat. No. 4,115,521, and U.S. Pat. No. 4,153,672, each of which was issued to Dunks, et al., relate to methods of synthesis of decaborane and methods of synthesis if B11H14−.
Although there have been reports in the literature for the synthesis of isotopically enriched boron, these synthetic routes are lengthy and often produce compounds in notably low yields. It thus would be desirable to have new methods to synthesize isotopically enriched borohydride and isotopically enriched boranes from isotopically enriched boric acid. It would be particularly desirable to have new methods to synthesize isotopically enriched MBH4, MB11H14, B10H14 and methods of preparing isotopically enriched large boranes of the formula, BnHm, (where n is between 12 and 96 and m≦n+8).